Gate valves generally have seat rings which provide the seats for the gate and which also engage with body surfaces surrounding the inlet and outlet passages through the body. In prior structure such seat rings have been made to be floating and have been installed in the body with a suitable spacer means to ensure that there is the desired clearance between the seat rings for movement of the gate without binding.
Examples of various types of prior structures used for maintaining the seat ring spacing in gate valves are disclosed in U.S. Pat. Nos. 4,188,016 (pair of opposed spacer carriers); 4,179,098 (spacer plates); 3,823,911 (spacer blocks); and 4,193,581 (a cage engaging lugs on the seat rings). All of such structures ensure the spacing for the gate and allow floating movement of the seat rings. The sealing of the seat rings against the body sealing surfaces is provided by resilient seals, such as O-rings.
In maintaining the seal of the seat in a gate valve, efforts have been made to utilize pressure of the fluid flowing through the valve to urge the seats in directions toward their respective body sealing surfaces. An example of such structure is disclosed in U.S. Pat. No, 4,487,393 in which the effective diameter of the seat seal against the body sealing surface is less than the effective diameter of the seal against the gate so that there is an effective pressure area exposed to the fluid pressure within the valve flow passages which urges the seats toward their body sealing surfaces. One disadvantage of such structure is that pressure which collects in the valve chamber works against this sealing force.
One of the difficulties encountered with such prior structures is washout of the seat rings which results from the collection of sand or other foreign particles between the seat ring and its body sealing surface.
Another problem encountered with prior gate valves has been that the seat rings are exposed to an unbalanced pressure which can urge the seat rings away from their body sealing surface and this allows entry of particles which can result in seat ring washout.